PayPal

Easier to use on the internet. I don't want to be typing in my credit card number when I'm not absolutely certain the site is secure. Paypal takes that risk away.

It's a very simple step for the retailer. It can be work to become able to accept credit cards (we had to do that for the robot site; it was a pain), whereas it's a lot easier for a retailer to hook up via Paypal.